Few-layer Bismuthene: Sonochemical Exfoliation, Nonlinear Optics and Applications for Ultrafast Photonics with Enhanced Stability

We propose a generalized equation to represent a continuum of surface reconstruction solutions of a given non-integrable gradient field. We show that common approaches such as Poisson solver and Frankot-Chellappa algorithm are special cases of this generalized equation. For a N x N pixel grid, the subspace of all integrable gradient fields is of dimension N 2 - 1. Our framework can be applied to derive a range of meaningful surface reconstructions from this high dimensional space. The key observation is that the range of solutions is related to the degree of anisotropy in applying weights to the gradients in the integration process. While common approaches use isotropic weights, we show that by using a progression of spatially varying anisotropic weights, we can achieve significant improvement in reconstructions. We propose (a) α-surfaces using binary weights, where the parameter a allows trade off between smoothness and robustness, (b) M-estimators and edge preserving regularization using continuous weights and (c) Diffusion using affine transformation of gradients. We provide results on photometric stereo, compare with previous approaches and show that anisotropic treatment discounts noise while recovering salient features in reconstructions.

What Is the Range of Surface Reconstructions from a Gradient Field

The performance and stability of organic–inorganic hybrid perovskite solar cells (PSCs) is sensitive to water and moisture in an ambient environment. Understanding how H2O influences the perovskite material is also important for developing appropriate control strategies to mitigate the problem. Here we provide a comprehensive review on the effect of water on the state-of-the-art lead-based perovskite solar cells in terms of perovskite material design, perovskite film preparation, device fabrication, and photovoltaic application. It is found that a moderate amount of water can facilitate nucleation and crystallization of the perovskite material, resulting in better perovskite film quality and enhanced PSC performance. The perovskite materials are irreversibly destroyed by H2O after a certain level of water, but they exihibit better tolerance than initially expected. Humidity resistant fabrication of high-performance PSC devices and modules should therefore be favoured. Generally, water shows a negative effect on the long-term stability and lifetime of PSCs. To reduce the effects from water during outdoor operation, attention should be paid to different protection methods such as varying the perovskite composition, optimizing the electron/hole transport layer and encapsulation of the device.

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Impact of H2O on organic–inorganic hybrid perovskite solar cells

A review on perovskite solar cells: Evolution of architecture, fabrication techniques, commercialization issues and status

Rapid improvement in photoconversion efficiency (PCE) of solution processable organometallic hybrid halide based perovskite solar cells (PSCs) have taken the photovoltaic (PV) community with a surprise and has extended their application in other electronic devices such as light emitting diodes, photo detectors and batteries. Together with efforts to push the PCE of PSCs to record values >22% – now at par with that of crystalline silicon solar cells – origin of their PV action and underlying physical processes are also deeply investigated worldwide in diverse device configurations. A typical PSC consists
of a perovskite film sandwiched between an electron and a hole selective contact thereby creating ESC/perovskite and perovskite/HSC interfaces, respectively. The selective contacts and their interfaces determine properties of perovskite layer and also control the performance, origin of PV action, open circuit voltage, device stability, and hysteresis in PSCs. Herein, we define ideal charge selective contacts, and provide an overview on how the choice of interfacing materials impacts charge accumulation, transport, transfer/
recombination, band-alignment, and electrical stability in PSCs. We then discuss device related considerations such as morphology of the selective contacts (planar or mesoporous), energetics and electrical properties (insulating and conducting), and its chemical properties (organic vs inorganic). Finally, the outlook highlights key challenges and future directions for a commercially
viable perovskite based PV technology.

/pdf/interfaces-in-perovskite-solar-cells-2gjofknguu.pdf

Interfaces in Perovskite Solar Cells

Interface degradation of perovskite solar cells and its modification using an annealing-free TiO2 NPs layer

We present a beam shaping technique in controlling the complex amplitude of an optical beam. The constraint on the amplitude of the output beam in the Gerchberg-Saxton algorithm is replaced with constraints both on the amplitude and phase of the output beam in the proposed method. The total areas of the constrained regions and free regions on the complex amplitude of the output beam in the proposed method are maintained. An output beam with arbitrary complex amplitude can be realized with the proposed method. The computing result from the proposed method is a phase-only distribution, which can be fabricated as diffractive optical element for higher diffraction efficiency. Both simulations and experiments are present and the effectiveness of the proposed method is verified.

Beam shaping of complex amplitude with separate constraints on the output beam.

One-dimensional semiconducting SnO2 nanowires with wide bandgaps are promising candidates to build many important optoelectronic devices. Because building these devices involves the assembly of nanowires into complex structures, manipulation of the active materials needs to be done with high spatial precision. In this paper, an optical tweezer system, comprising a spatial light-modulator, a microscope, and optical elements, is used to individually trap, transfer, and assemble SnO2 nanowires into two-terminal photodetectors in a liquid environment. After the assembly using optical trapping, the two ends of the SnO2 nanowire photodetectors, which are connected with the electrodes, were further stabilized using a focused laser. During exposure to 275 nm deep-ultraviolet light, the as-assembled photodetectors show a high Iph/Idark ratio of 2.99 × 105, a large responsivity of 4.3 × 104 A W−1, an excellent external quantum efficiency of 1.94 × 105, and a high detectivity of 2.32 × 1013 Jones. The photoresponse-speed of the devices could be improved further using passivation with a polymer. The rise and decay times are about 60 ms and 100 ms, respectively. As a result of this study, we can confirm that non-contact optical trapping can enable the construction of nanowire architectures for optoelectronic, bioelectronic, and other devices.

High-performance solar-blind SnO2 nanowire photodetectors assembled using optical tweezers

This paper describes the tri-phase all-optical switching and broadband nonlinear optical response in Bi2Se3 nanosheets. Using Bi2Se3 nanosheets dispersion solution as the sample, the spatial phase of controlled light can be modulated as three phases (unchanging, focusing, diffraction) by changing the incident intensity of controlling light. The mechanism is conjectured that the controlling light changes the phase distribution of overlapping region and then modulates the phase distribution of the controlled light. Based on Gerchberg-Saxton algorithm, the phase distribution of the controlling light and controlled light is retrieved from the transmitted patterns. In dynamic spatial self-phase modulation (SSPM) experiment, the three processes including self-focusing, self-diffraction ring formation, and self-diffraction ring deformation can also be observed. In addition, the SSPM of controlling light is measured at the typical wavelengths from 350 nm to 1160 nm, which demonstrates that this all-optical switching is available in broadband. These results provide the great potential of Bi2Se3 as an all-optical switching for various optoelectronic applications.

Tri-phase all-optical switching and broadband nonlinear optical response in Bi 2 Se 3 nanosheets.

We observe dynamic self-diffraction in MoS2 supernatant solutions with laser for the first time, and conduct dynamic data simulation and analysis. Observation results indicate that self-diffraction can be divided in three stages: in the first stage, laser changes from Gauss beam to symmetric diffraction rings because of the force from laser. In the second stage, diffraction rings become asymmetric vertically because of gravity. In the third stage, diffraction rings become asymmetric horizontally, as a result of fine structure of laser. We obtain the dynamic distribution of MoS2 nanoflake in solution under the effect of laser by dynamic diffraction image simulation.

Shaohua Tao

Papers

Interface degradation of perovskite solar cells and its modification using an annealing-free TiO2 NPs layer

Beam shaping of complex amplitude with separate constraints on the output beam.

High-performance solar-blind SnO2 nanowire photodetectors assembled using optical tweezers

Tri-phase all-optical switching and broadband nonlinear optical response in Bi 2 Se 3 nanosheets.

Dynamic self-diffraction in MoS(2) nanoflake solutions.